I was watching 4 chickens for a friend not too long ago, and she asked me to change the water every other day (2 gallon waterer). In two days they were down to the dregs (but not out).
BUT--and this is important--that'll change depending on the temperature and other things.
well I change their water like 4 times a day right now LOL! I figured that in the winter it would be harder to change all the time with the loads of snow that we get!!